This invention relates to an airflow regulator, and more particularly to a passive aspirator. It is especially intended for use in an automotive climate control system and will be described in connection with that application. It will be understood that the invention is not limited to automotive use but could be used in any passive aspirator, regardless of the application.
Automatic climate control systems for automotive vehicle passenger compartments require several inputs of information. Among these is the current temperature of the air in the compartment. The temperature information is used to regulate the vehicle's heating, ventilating and air conditioning system. To obtain an accurate reading of the temperature, it is necessary to cause the movement of a representative sample of vehicle interior air across a temperature sensor. The sensor is typically a thermistor (a negative temperature coefficient resistor) mounted in an air passage or duct. The air movement can be created by an active or passive aspirator. An active aspirator uses a small electrical fan to create the pressure differential which causes air to flow past the thermistor. A passive aspirator uses the Venturi effect to create the pressure difference from a positive pressure source. This pressure difference, in either case, causes the required air flow across the sensor.
A typical passive aspirator of the type known in the prior art is capable of moving 0.2 to 0.5 ft.sup.3 /min. (cfm) of air past the thermistor. It is a matter of debate whether this is sufficient air flow to provide an accurate temperature reading. Some skilled in the art believe that a higher air flow is desirable to obtain a better temperature sample and, therefore, better regulation of vehicle interior temperature.
One way to achieve higher air flow is, of course, to use an active aspirator with its electric fan but this approach significantly adds to the cost of the system. Another way to achieve higher air flow without the cost of an active aspirator is to use a larger passive aspirator but doing so leads to other problems. Scaling up the size of the aspirator results in increasing the amount of air flowing through the device. This is waste air which is dumped somewhere under the vehicle instrument panel. Waste air generally has not been heated or cooled to the desired temperature, nor is it directed through the user-selected vents or ducts. While some waste air can be tolerated, excessive amounts are considered detrimental to occupant comfort. This limit on waste air, as well as physical constraints within a vehicle, restricts the size of a passive aspirator.
Sizing a passive aspirator is further complicated by the fact that the positive air pressure source is typically the main fan for the vehicle temperature control system. The main fan speed can be selected by the vehicle occupants at anywhere from 0 to 100% of maximum. Thus, the positive pressure applied to the aspirator can vary widely. An aspirator large enough to generate, say, 1 cfm of flow across the thermistor at a fan speed of 20%, will probably produce excess waste air at a fan speed of 80%. Conversely, an aspirator small enough to avoid excessive waste air at a fan speed of 80% of maximum will generate too small a flow at 20% fan speed. The present invention overcomes these difficulties by providing a passive aspirator having a variable flow volume dependent on the level of positive pressure at the aspirator inlet.